This invention relates to the structure and operation of electromechanical modules of the type which include an integrated circuit package (hereinafter IC package) and a heat sink that is mechanically pressed against the IC package. By an IC package is herein meant, an enclosure which contains one or more integrated circuit chips and has input/output terminals for sending/receiving electrical signals to/from the enclosed chips. Typically, the enclosure is made of ceramic, or plastic, or an epoxy glass.
In a Pin Grid Array IC Package, the input/output terminals are an array of metal pins which extend perpendicular to the enclosure and get soldered into plated through holes in a printed circuit board. In a Cast Column Grid Array IC Package, the input/output terminals are an array of cast metal columns which extend perpendicular to the enclosure and get soldered to signal pads on the surface of a printed circuit board. In a Ball Grid Array IC Package, the input/output terminals are an array of metal balls which get soldered to signal pads on the surface of a printed circuit board. These metal balls can be made of a metal which stays solid when the solder melts, or the metal balls can be made of the solder itself.
One way in which a heat sink has been attached to an IC package in the prior art is with an epoxy or solder. However, a problem occurs with such an attachment if the IC package and the heat sink are made of materials which have substantially different thermal expansion rates. In that case, the epoxy/solder joint and the IC package can become stressed and crack when the IC package is subjected to temperature cycles during normal operation.
Also when the heat sink is soldered/epoxied to the IC package, another problem arises when the IC package becomes defective and needs to be de-soldered from the printed circuit board. In that case, the heat sink on the IC package will draw heat away from the input/output terminals during a desoldering operation; and consequently, the task of melting the solder on the input/output terminals becomes difficult. If too much heat is used, one or more input/output terminals on an adjacent IC package can accidently become de-soldered.
To avoid the above problems, several electromechanical modules have been disclosed in the prior art in which a heat sink is mechanically pressed against the IC package. This allows the heat sink and the IC package to slide against each other during temperature cycling in normal operations; and it allows the heat sink to be easily removed from the IC package before a de-soldering operation. Such modules are described in the following U.S. Pat. Nos. 4,719,494 to Bright, et al.; U.S. Pat. No. 5,208,731 Bloomquist; U.S. Pat. No. 4,679,118 to Johnson, et al.; U.S. Pat. No. 4,745,456 to Clemens; U.S. Pat. No. 5,019,940 to Clemens; and U.S. Pat. No. 5,307,239 to McCarty, et al. However, in all of the above patents, the electromechanical modules which are described have other drawbacks.
In the first three patents, '494 and '731 and '118, the electromechanical module includes a socket which has input/output terminals that get soldered into a printed circuit board. Then, the input/output terminals of the IC package are mechanically pressed against corresponding contacts in the socket. Also, the socket provides a mechanism by which the heat sink is mechanically pressed against the IC package.
However, a problem with the above structure is that the input/output terminals of the IC package itself are not soldered. Instead, only mechanical connections are made between the input/output terminals of the IC package and the contacts in the socket; and such mechanical connections are less reliable than soldered connections.
Another problem with the above sockets is that the mechanical connections between the input/output terminals on the IC package and the socket contacts take more space than a corresponding number of soldered connections. Consequently, the size of the IC package must be made larger than it could otherwise be if the input/output terminals were soldered directly into the printed circuit board.
Still another problem with the above sockets is that they add parasitic inductance to the input/output terminals of the IC package. This inductance arises from the mechanical contacts on the socket, the input/output terminals on the socket which got soldered into the printed circuit board, and their interconnections within the socket. Such inductance is undesirable because it slows down the speed with which signals can be sent to/received from the IC package.
In the next two patents, '940 and '456, the electromechanical module which is described contains no socket, and the input/output terminals of the IC package are soldered directly into the printed circuit board. This is achieved by providing a heat sink frame (hereinafter frame) which surrounds the IC package and attaches to the perimeter of the IC package. Input/output terminals of the IC package pass through the surrounding frame and get soldered directly to the printed circuit board; and a spring attaches to the frame and presses the heat sink against the IC package.
However, a problem with these two electromechanical modules is that the frame cannot be removed from the IC package prior to a de-soldering operation in which a defective IC package is to be removed from the printed circuit board. Thus, the frame must continue to surround the IC package while it is de-soldered from the printed circuit board. This is undesirable because such de-soldering is often accomplished by passing a focused stream of hot gas past the input/output terminals of the IC package in order to melt the solder; and the presence of the frame around the IC package will impede the flow of the gas. Consequently, one or more input/output terminals can heat up and get de-soldered on another IC package on the printed circuit board which lies adjacent to the IC package that is to be removed.
Another problem with the above electro-mechanical modules is that the frame must be attached to the IC package before the IC package is soldered into the printed circuit board. This limitation makes the module unsuitable for use with surface mounted IC packages, like the previously described Cast Column Grid Array IC package and Ball Grid Array IC package. When a surface mounted IC package is soldered to the printed circuit board, the surface tension in the melted solder is used to precisely move the IC package such that input/output terminals align with the signal pads on the printed circuit board. But, the presence of a frame around the IC package will hinder this movement and can cause misalignment between the input/output terminals and the signal pads.
In remaining patent '239, the above two problems are overcome in that an electromechanical module is disclosed which includes a frame that holds a heat sink against an IC package; and, the frame can--a) be attached to the IC package after the IC package is soldered to the printed circuit board, and b) be removed from the IC package before the IC package is de-soldered from the printed circuit board. However, in patent '239, the frame is structured such that it must rest on top of the IC package in the completed module in order to be held in place; and the frame must have room to slide on top of the IC package in order to be attached or removed. Consequently, none of the area on the top of the IC package which is used by the frame can be contacted by the base of the heat sink, and this limitation hinders the extent to which the IC package can be cooled.
In addition, in patent '239, the frame will block the flow of air past the cooling fins on the heat sink unless the base of the heat sink is taller than the frame. Thus, the frame limits the minimum profile of the complete electromechanical module. However, any increase in the height of the module has a serious ramification in high speed digital computer systems where multiple printed circuit boards which hold the modules are mounted side-by-side on a backplane. There, the increased height of the module will increase the board-to-board spacing on the backplane; and that in turn will make the signal lines longer on the backplane and cause the system to run at a slower speed. For example, a 20% increase in the spacing of the printed circuit boards which causes a corresponding decrease in the operating speed of a high speed computer system would be disastrous to the marketability of the product.
This speed problem is made worse in patent '239 when, in order to cool the IC package sufficiently, the heat sink base needs to be so large that it extends past the frame. In that case, the heat sink base must include a pedestal in order to get over the frame. But such a pedestal in the heat sink base further increases the height of the complete module, which in turn increases board-to-board spacing and decreases operating speed.
Accordingly, a primary object of the invention is to provide a novel and improved electromechanical module in which all of the above-described drawbacks are overcome.